The present invention pertains to a large-aperture, high-magnification [i.e., high-zooming-ratio] zoom lens.
There have heretofore been a great many proposals of zoom lenses having, in order from the object-most side, positive-negative power distributions, these including, for example, Japanese Laid-Open Patent Application (Kokai) No. S57[1982]-168209 and the like. Moreover, there have been a number of proposals of large-aperture, high-magnification [i.e., high-zooming-ratio] zoom lenses, these including, for example, Japanese Laid-Open Patent Application (Kokai) No. H4[1992]-208911 and Japanese [Laid-Open] Patent [Application (Kokai)] No. H4[1992]-208912. The zoom lenses disclosed in [these] Japanese Laid-Open Patent Applications have field angles of 62.degree. and zooming ratios of approximately 3.times., and have an F-number of F 2.8 at all focal lengths. Furthermore, a large-aperture zoom lens having a positive-negative-positive-positive-positive five-group constitution has been proposed at Japanese Laid-Open Patent Application (Kokai) No. H6[1994]-281862.
However, the aperture of the zoom lens disclosed at Japanese Laid-Open Patent Application (Kokai) No. S57[1982]-168209 is small, at [only] F 3.5 to F 4.5, and moreover, the field angle coverage thereof extends to only on the order of 62.degree.. Furthermore, from the standpoint of aberrations as well, as there is incomplete correction of fluctuations in spherical aberration and astigmatism occurring as a result of zooming, and as some fluctuation in coma also remains, it would be difficult to achieve a large-aperture, wide-angle zoom lens having a field angle greater than 75.degree. and [an aperture] on the order of F 2.8 with this optical system, given the constitution and distribution of refractive power therein [i.e., as disclosed]. Furthermore, while the large-aperture zoom lenses disclosed in Japanese Laid-Open Patent Application (Kokai) No. H4[1992]-208911 and Japanese Laid-Open Patent Application (Kokai) No. H4[1992]-208912, and in Japanese Laid-Open Patent Application (Kokai) No. H6[1994]-281862, attain satisfactory levels with respect to correction of aberration, and have large apertures, at F 2.8, as field angle at the wide-angle end is only on the order of 62.degree., and as zooming ratio is on the order of [only] 2.5.times. to 3.times., further increases in field angle and in zooming ratio are desired.
The object of the present invention is therefore to solve the aforementioned problems and to provide a high-performance zoom lens having a wide field angle and a zooming ratio greater than 3.5.times., and having a large aperture such that F-number is on the order of F 2.8.